The present invention relates to a method for treating an oil and water mixture of continuous phase and dispersed phase so as to facilitate subsequent separation of the phases. The intent of the present invention is to increase the average size of dispersed phase particles in the mixture under treatment.
This invention is primarily concerned with recovering bitumen or oil phase from oil and water mixtures produced from oil wells and oil or tar sands. Extensive deposits of oil sands, which are also known as tar sands and bituminous sands, are found in Northern Alberta, Canada. The sands are composed of siliceous material with grains generally having a size greater than that passing a 325 mesh screen (44 microns) and a relatively heavy, viscous petroleum called bitumen which fills the voids between the grains in quantities of from 5 to 21 percent of total composition. (All percentages referred to herein are in weight percent unless noted otherwise). Generally the bitumen content of the sand is between 5 and 15 percent. This bitumen contains typically 4.5 percent sulfer and 38 percent aromatics. Its specific gravity at 60.degree. F. ranges generally from about 1.00 to about 1.06. The oil sands also contain clay and silt. Silt is defined as siliceous material which will pass a 325 mesh screen, but which is larger than 2 microns. Clay is material smaller than 2 microns including some siliceous material of that size. Extensive oil sand deposits are also found elsewhere in the world such as in the Orinoco heavy oil belt of Venezuela and in the area near Vernal, Utah. The mineral and bitumen of these deposits differ somewhat from those of the Alberta deposits. Compared with the Alberta oil sands, the Utah deposit contains a coarser sand, less clay and an even more viscous bitumen.
Much of the world resource of bitumen and heavy oil is deeply buried by overburden. For example, it has been estimated that only about 10 percent of the Alberta oil sand deposit is close enough to the earth's surface to be conveniently recovered by mining. The remainder is buried too deeply to be economically surface mined. Hydraulic mining or tunnel mining has been proposed for these deeper deposits. Generally, however, it is considered that enhanced recovery by steam injection, by injection of aqueous solutions, and/or by in situ combustion may possibly be more effective for obtaining bitumen or heavy oil from deeply buried formations. Such enhanced recovery methods use one or more oil wells that penetrate the formation and stimulate or recover the resource. Recovery of bitumen from a well by steam stimulation is described in Canadian Pat. No. 822,985 granted on Sept. 16, 1969 to Fred D. Muggee. Depending upon the procedure employed, enhanced recovery methods either produce mixtures of oil, water and water-in-oil emulsions or produce oil-in-water emulsions.
There are several well-known procedures for separating bitumen from mined oil sands. In a hot water method such as disclosed in Canadian Pat. No. 841,581 issued May 12, 1979 to Paul H. Floyd, et. al., the bituminous sands are jetted with steam and mulled with a minor amount of hot water and sodium hydroxide in a conditioning drum to produce a pulp which passes from the conditioning drum through a screen which removes debris, rocks and oversize lumps to a sump where it is diluted with additional water. It is hereafter carried into a separation cell.
In the separation cell, sand settles to the bottom as tailings which are discarded. Bitumen rises to the top of the cell in the form of a bituminous froth which is called the primary froth product. An aqueous middlings layer containing some mineral and bitumen is formed between these layers. A scavenging step is normally conducted on this middlings layer in a separate flotation zone. In this scavenging step, the middlings are aerated so as to produce a scavenger tailings product, which is discarded, and a scavenger froth product. The scavenger forth product is thereafter treated to remove some of its high water and mineral matter content and is thereafter combined with the primary froth product for further treatment. This combined froth product typically contains about 52 percent bitumen, 6 percent minerals, 41 percent water, all by weight, and may contain from 20 to 70 volume percent air. It resembles a liquid foam that is difficult to pump and, for that reason, is usually treated with steam to improve its flow characteristics.
The high water and mineral contents of the combined froth product normally are reduced by diluting it with a hydrocarbon diluent such as naphtha. It is then centrifuged to produce a tailings product and a final bitumen product that typically contains essentially no water and about 1.3 percent solids and that is suitable for coking, hydrovisbreaking and other refining techniques for producing a synthetic crude oil. The tailings products, containing some naphtha, are discarded.
There are basically four effluent streams from the Hot Water Process. Each carries with it some of the bitumen of the feed thereby reducing the efficiency of the process. These include the oversize material, the sand from the separation cells, the silt and clay from the scavenger cells and the tailings from the centrifuges. Up to 10 percent of the bitumen in the original feed and up to 21/2 percent of the naphtha stream may be lost in this manner. Much of this bitumen effluent finds its way into large retention ponds that are typical of the Hot Water Process. The bottom of one such retention pond may contain up to 50 percent dispersed mineral matter substantially of clay and silt as well as 5 percent bitumen. As disclosed in Canadian Pat. No. 975,697 issued on Oct. 7, 1975 to Davitt H. James this part of the pond contents, referred to as sludge, is a potential source of bitumen.
The Hot Water Process described in the preceding paragraphs separates bitumen from a prepared oil sand slurry. Various methods for preparing oil sand slurries are taught in the prior art, as for example disclosed in Canadian Pat. No. 918,588 issued on Jan. 9, 1973 to Marshall R. Smith, et. al., and in U.S. Pat. No. 3,968,572 issued on July 13, 1976 to Frederick C. Stuchberry. These apparatus as disclosed were especially designed to form a slurry that is hot, that contains finely dispersed air bubbles and wherein the bitumen is in the form of small flecks. Such a slurry is amenable to subsequent separation in a hot water bath after dilution wherein bitumen forms into a froth that rises to the top of the bath and is skimmed therefrom. Alkaline reagents such as sodium hydroxide are normally added in this process to give to the slurry those properties that provide for efficient flotation of the bitumen in said water bath. However, in the presence of sodium hydroxide, fine clay particles in the effluent streams from this process do not settle readily. For this reason inordinately large settling ponds are required to contain the effluents from commercial hot water oil sands extraction plants.
The present invention applies to a method of treating various streams from oil sand operations having a dispersed oil or aqueous phase to cause combination of dispersed particles, which combination improves the recovery of the oil phase by the use of apertured oleophilic endless conveyor belts to achieve oil phase-aqueous phase separations. These processes are superior to the Hot Water Process because separations are conducted at lower process temperatures and with lower water requirements. For comparable oil sand feedstocks, the bitumen produced by combination of dispersed phase particles followed by oil phase-aqueous phase separation with an apertured oleophilic belt as typically disclosed is of higher quality than the froth produced by a Hot Water Process.
The apertured oleophilic conveyor belt, that may be used to separate emulsions, slurries, or mixtures of oil phase and aqueous phase, typically consists of a mesh belt that is woven from fibre, string or wire of high tensile strength and fatigue resistance, that is oleophilic by nature or that will bond strongly with a belt coating that is oleophilic. This belt typically is supported by two conveyor end rolls that provide tension and form to the belt. Separation is achieved by passing a slurry, emulsion or mixture of oil phase and water phase, with or without particulate solids, through the belt one or more times. Water phase and particulate solids in the water phase pass through the belt apertures and are discarded while oil phase attaches itself to the belt because of its attraction for the oleophilic belt surfaces. The oil phase subsequently is recovered from said belt as a product. Typical processes are disclosed in U.S. Pat. No. 4,224,138 and U.S. Pat. No. 4,236,995 and copending U.S. patent application, Ser. No. 178,000.